PROJECT SUMMARY While conventional cigarette use continues to decline among youth and young adults, e-cigarette (EC) use is on the rise. The use of ECs during young adulthood, particularly 18 years of age, is especially alarming because it is not only a critical period in development but also a time when tobacco use is established. Additionally, the tobacco industry targets individuals of this age with the hope that they will one day progress to using combustible cigarettes. Advertising may be one of the reasons leading young people to use ECs, and the Food and Drug Administration (FDA) now has the authority to regulate EC advertisement features. The goal of the proposed two-phase study is to determine which EC ad features most strongly influence young adults' attitudes, susceptibility, and intentions to use ECs (K99), and determine if the restriction of these ad features influences EC attitudes, experimentation, and sustained use (R00). In phase 1 (K99), we will select 15 ads from the most popular EC brands that employ a brand, product descriptions, and modeling features. Young adults who are susceptible to EC use will come into the lab and view these ads. During exposure, they will be assessed for real-time visual attention using eye-tracking, orienting responses using heart rate, and arousal using skin conductance as well as pre- and post-ad self-report measures of attitudes, susceptibility, and behavioral intentions. These factors will help determine the most high impact features, which will be associated with the greatest visual attention, orienting responses, and arousal levels and changes in attitudes and will be targeted for restriction in the regulated ad arm of the R00. In phase 2 (R00), we will experimentally estimate the causal effect of restricting high impact EC ad features on tobacco use behavior. Specifically, we will conduct a randomized control trial (RCT) where young adults are randomized to one of two conditions: 1) a regulated ad environment, with high impact EC ad features removed or 2) an unregulated ad environment, with high impact EC ad features left intact. Participants will view EC ads embedded in an online magazine 24 times over the course of 6 months. EC experimentation and sustained use will be the primary outcome of interest. Findings from this study will provide public health officials important and urgently needed information as to what advertising features are contributing to the sharp rise in the use of ECs among young adults, and if restrictions reduce the use of EC among young adults.